Matrix displays for displaying visual images include active and passive matrix liquid crystal displays (LCDs), light emitting diode (LED) displays, electro-luminescent (EL) displays, and field emission displays (FEDS). Essentially, a matrix display is established by a grid consisting of co-parallel column electrodes that are perpendicularly juxtaposed with co-parallel row electrodes, with the intersections of the electrodes defining pixels. The intensity of each pixel is established by appropriately establishing the voltage difference between the corresponding electrodes that define the pixel. When properly arranged by means of controlling the voltages imposed on the pixels, the combination of lighted and unlighted pixels establishes the image sought to be presented.
Most matrix displays use what is essentially multiplexing in establishing the voltage (and, hence, intensity) for each pixel. More specifically, a single frame of a matrix display (which can represent a single still image) ordinarily is established by sequentially enabling the rows of pixels, i.e., illuminating the rows of pixels one at a time starting at the top row and working down row by row to the bottom row.
To enable a row, the row is energized with a "select" voltage which enables each pixel in the row to be excited when a relatively high "on" voltage is applied to its corresponding column electrode. A pixel will remain substantially unexcited, however, when a relatively low "off" voltage is applied to its corresponding column electrode. In contrast, the non-enabled rows are energized with a "suppress" voltage, which prevents the pixels in the rows from being excited regardless of the voltage of the column electrodes. Accordingly, with this scheme the voltages of the column electrodes are established as appropriate for generating the portion of the desired image which is to be produced by whichever row is enabled.
Preferably, to avoid visual artifacts and particularly to avoid flickering in video/animation (i.e., moving) presentations, the individual still images that define the video presentation are displayed and regenerated quickly, typically in 1/30 of a second. By updating matrix displays, i.e., by regenerating the still images that together establish a video presentation, at thirty Hertz (30 Hz), a video display consisting of successively presented still images can be presented. Accordingly, it will readily be appreciated that the larger the matrix display (many displays have 480 rows and 640 columns or more) and the faster the frames are to be regenerated, the shorter the time available to excite, i.e., to drive, each pixel.
With short drive periods, control of the display is made more difficult. It happens, however, that display control is important in causing the display to present not just pure black and white images (corresponding to pixels being either on or off), but to also display various shades of gray, termed herein "grayshading". Effective grayshading results in better, more realistic-appearing images.
Not surprisingly, past efforts have been made to provide for grayshading of matrix displays. Generally, these past efforts have either required spatial dithering or temporal dithering, also referred to as frame rate control.
In spatial dithering, perceptions of various levels of gray are achieved by grouping pixels and illuminating the individual pixels in a group as required to achieve an overall gray shade for the group. In other words, spatial dithering recognizes that the human eye will integrate the blackness of various pixels in a small group of pixels with the whiteness of various other pixels in the group to perceive the desired shade of gray. Unfortunately, one drawback of spatial dithering is that display resolution is reduced, because the smallest individual unit of display effectively is no longer a single pixel, but a single group of pixels.
In contrast to spatial dithering, which averages the simultaneous appearance of a group of pixels, frame rate control averages the appearance of individual pixels over time. Thus, in a simple example, a single image might be established by two frames instead of one, making possible three shades for each pixel of the image. More specifically, in this simple example a pixel could be perceived as white, if the pixel is white for both frames, or black, if the pixel is black for both frames, or gray, if the pixel is white for one frame and black for the other frame. Because the eye integrates the appearance of the pixel, under current frame rate control it makes no difference whether a gray pixel is black or white during the first frame, as long as it assumes the opposite shade during the second frame.
Thus, frame rate control systems using "n" frames per image must generate the frames at "n" times the desired image regeneration frequency. Unfortunately, it happens that in many types of matrix displays, e.g., LCDs, the pixels cannot instantly be turned from "on" to "off", and require a finite relaxation time to essentially deenergize, making extremely rapid update rates difficult to achieve and control. Compounding this problem is the multiplexing characteristic of matrix displays discussed above, wherein the duty cycle of a pixel (i.e., the time available to energize the pixel) is a small fraction of the total number of rows. Accordingly, either the number of frames per image must be limited, thereby limiting the possible number of levels of grayshading, or the image regeneration rate must be slowed, thereby leading to display artifacts such as flicker, particularly when the presented image is changing.
As a variation of frame rate control, previous methods have modulated either the pulse height or pulse width of the voltage applied to the column electrodes, thereby modulating the overall intensity of the pixel (and, hence, establishing an apparent shade of gray). More specifically, with the enabled row electrode being energized with the "select" voltage, the column electrodes can be energized with various combinations of intensities or pulse widths of "on" voltages. While such methods can increase the possible number of grayshading levels, it remains difficult to precisely control grayshading by multiple pulsings of pixels, in light of large matrix sizes, rapid update rates, and the consequent low duty time of each pixel.
Accordingly, it is an object of the present invention to provide a system and method for establishing relatively many levels of grayshading in a matrix display, without unduly slowing the frame regeneration rate of the display. Another object of the present invention is to provide a system and method for establishing relatively many levels of grayshading in a matrix display which can relatively easily be backfit into existing displays. Yet another object of the present invention is to provide a system and method for establishing relatively many levels of grayshading in a matrix display which is controllable with comparatively high precision. Still another object of the present invention is to provide a system and method for establishing grayshading in a matrix display which is easy to use and cost-effective. Another object of the present invention is to provide a system and method for establishing grayshading in a matrix display which reduces artifacts in the presented image when the image is changing.